231- OPTICS. 



or spreading out from a single point in the luminous 

 body. 



Shadows are occasioned merely by the ob- 

 struction of light by an opaque body. 



OF REFRACTION. 



If the rays of light, after passing through a 

 medium, enter another of a different density, in a 

 direction perpendicular to its surface, they proceed 

 through this medium in the same direction as 

 before. Thus, the ray F C (Plate 12. n'g. 1.) pro- 

 ceeds to k, in the same direction. 



But if they enter obliquely to the surface of a 

 medium, either denser or rarer than what they 

 moved in before, they are made to change their 

 direction in passing through that medium. 



If the medium which they enter be denser, 

 they move through it in a direction nearer to the 

 perpendicular drawn to its surface. Thus, B C, 

 upon entering the denser medium L G, instead of 

 proceeding in the same direction G H, is bent into 

 the direction C I, which makes a less angle with 

 the perpendicular C K. 



On the contrary, when light passes out of a 

 denser into a rarer medium, it moves in a direction 

 farther from the perpendicular. Thus, if I C were 

 a ray of light which had passed through the dense 

 medium L G, on arriving at the rarer medium, it 

 would move in the direction C B, which makes a 

 greater angle with the perpendicular. 



This refraction is greater or less, that is, the rays 

 are more or less bent or turned aside from their 

 course, as the second medium through which they 

 pass is more or less dense than the first. 



